Goals: Seventeen years after the successful Viking 1 and Viking 2 missions, Mars Observer was launched to make a detailed study of the Red Planet. The spacecraft, based on an Earth-orbiting communications satellite, which had been converted into an orbiter for Mars, carried a payload of science instruments designed to study the geology, geophysics and climate of Mars. The journey took 337 days.

Accomplishments: The spacecraft was about to begin pressurizing its fuel tanks in preparation for an orbit-insertion maneuver when its transmitters were turned off and the spacecraft was never heard from again. Before contact with the spacecraft was lost, approximately two months of total data from the gamma ray spectrometer was successfully collected, including spectral observations of one burst.